1. Field of the Invention
This invention relates to an apparatus for effecting at least one of recording and reproduction of information by applying a light beam to a recording medium, and in particular to an optical information processing apparatus provided with means for effecting the tracking or focusing control of the light beam and means for detecting the abnormality of said control.
2. Related Background Art
Various shapes such as a disc-like shape, a card-like shape and a tape-like shape are known as the forms of a medium on which a light is used to record information and read the recorded information. These optical information recording mediums include ones which are capable of recording and reproduction and ones which are capable of only reproduction.
Recording of information on a medium capable of recording is accomplished by scanning information tracks by a light beam modulated in accordance with recording information and stopped down into a minute spot-like shape, and information is recorded as an optically detectable information bit row.
Also, reproduction of information from a recording medium is accomplished by scanning the information bit row of an information track by a light beam spot of such a degree of predetermined power that recording is not effected on the medium, and detecting the light reflected from or transmitted through the medium.
In an apparatus for information recording onto the recording medium as described above and information reproduction from such recording medium, a so-called optical head is used for the application of a light beam spot to the recording medium and for the detection of the light reflected from or transmitted through the medium. The optical head is movable relative to the recording medium in the information track direction thereof and in a direction transverse to said track direction, and the scanning of the information tracks by the light beam spot is accomplished by said relative movement. Also, in the optical head, a portion of an optical system, for example, an objective lens, is held for independent movement relative to the optical head body in the direction of the optic axis thereof (the focusing direction and in a direction orthogonal to both the direction of the optic axis and the information track direction of the recording medium (the tracking direction). Holding of the objective lens is generally accomplished through a resilient member, and the movements of the objective lens in said two directions are generally driven by an actuator utilizing an electromagnetic interaction.
Among the optical information recording mediums as described above the card-like optical information recording medium (hereinafter referred to as the "optical card") has a great estimated demand as an information recording medium of relatively great capacity which is compact and light in weight and convenient for carrying.
FIG. 1 of the accompanying drawings is a schematic plan view of a postscript type optical card, and FIG. 2 of the accompanying drawings is a fragmentary enlarged view thereof.
In FIG. 1, a number of parallel information tracks 4 extending in a direction LF are arranged on the information recording surface of the optical card 1. Also, a home position 3 which provides the reference position for the access to the information tracks 4 is provided on the information recording surface of the optical card 1. The information tracks 4 are arranged in the order of 4-1, 4-2, 4-3, . . . in succession from the home position 3. The information tracks 4 include two kinds of tracks, i.e., tracks on which information is already recorded and tracks on which information is not yet recorded. Information can be recorded at any time on the information tracks on which information is not yet recorded.
As shown in FIG. 2, tracking tracks 5 (for example, 5-1, 5-2, 5-3) are provided between adjacent tracks 4 (for example, 4-1 and 4-2). These tracking tracks are utilized as a guide for auto tracking (AT) which controls a light beam spot so as not to deviate from a predetermined information track when it scans during the information recording and reproduction. This AT servo is accomplished by detecting the deviation of the light beam spot from an information track 4 (an AT error) in an optical head, negatively feeding back the detection signal to the tracking actuator, moving an objective lens relative to the optical head body in the tracking direction and causing the light beam spot to follow a desired information track.
Now, if there is a great defect on the medium or a shock is applied to the apparatus when information is recorded or reproduced as described above, there may occur out-of-AT servo. In such case, usually, the AT servo loop is once opened and after the defect or the like has been passed, the AT servo is again drawn in. In this method, however, the position of the objective lens while the servo loop remains opened cannot be known, and this has led to the problem that during the re-draw-in, much time is required for the beam spot to return to the initial track in which the out-of-AT servo has occurred. Also, no drive force is applied to the objective lens while the servo loop remains opened and therefore, there has been the danger that due to vibration, the objective lens collides against the frame of the optical head and is damaged thereby.
On the other hand, in Japanese Laid-Open Patent Application No. 59-142757 (laid open on Aug. 16, 1984), there is proposed a method of detecting the defect of said medium and holding a tracking signal. However, again in this method, if positional deviation occurs during the holding, re-draw of AT into the initial track does not always takes place after the holding is released. Accordingly, in some cases, much time has also been required before the beam spot returns to the initial track.
Also, the problem of out-of-servo as described above has arisen not only in AT servo, but also in auto focusing (AF) servo for accurately focusing the light beam onto the medium surface.